The invention relates to a method and system for contactless exchange of information between a read/write unit and at least one information carrier, using wireless radiofrequency signal transmission, for reading and writing respectively of data in the information carrier(s), said unit comprising an input/output circuit and associated coil means and the information carrier comprising another input/output circuit and associated coil means, and each further comprising a processing circuit, both coil means being inductively coupled to each other for bidirectional data transmission when the information carrier is positioned within range, whereby the coil means in the information carrier, solely during transmission of the radiofrequency signal by the read/write unit, are detuned or not for said data transmission. Such a method and system are known from EP-A-0461878.
These systems, known from said publication and in practice, are used for many purposes. In the following said read/write unit will be denoted with the designation "reader". Furthermore, in this description, when a chip card is mentioned as an example of an information carrier it should be clear that herewith also a chip pen or such element is meant. With such a reader and associated information carrier many kernel functions can be implemented, such as
establishing whether card and reader are authentic; PA1 establishing the identity of the card holder, which can be both a human being and an object or good; PA1 granting of authorization or key function to the card holder, both physical authorization (e.g. access to a building or space), and logic authorization (e.g. access to a service or data file); PA1 supporting a transaction, e.g. for a payment, supply of a service or for a telephone call; PA1 storage of data, e.g. data of the card holder and of management data required for a good working of the application.
During said contactless exchange of information, data is transmitted via radiofrequency transmission. The information carrier therefore is provided with a circuit which has its own power supply, or with a circuit which derives its energy from the radiofrequency electromagnetic field, supplied by the reader, whereby in the last case the associated memory may have its separate power supply.
Said contactless information carriers will fulfil in the future a key role in several industrial and commercial processes in which large numbers of persons and/or goods have to pass. This is for example the case for public transport, distribution centers and on toll high ways. The access in all these processes, the validation and control by means of contactless cards is faster and for the system in its whole cheaper in exploitation.
The systems known in practice therefore are provided in general both in the reader and in the information carrier with an input/output circuit which for the emission and for receipt of data signals has separate coil means. In practice, these systems have a relative large sensitivity for interference specifically in surroundings having moving or static (metal) objects and with dirt and dust. Moreover, the implementation in the information carrier with separate coil means for transmitting and for receiving its rather cumbrous.
Even in the system known from above mentioned European application, in which the information carrier uses a single coil means both for transmitting and receiving, the sensitivity for interference is rather high. Furthermore, in this known system, for data transport in both directions, the ones and zeros are distinguished by the number of high- and low-interchanges in a timeslot, i.e. in the period between two synchronization pulses. These interchanges are determined by a comparator being fixedly adjusted, through which the operational range is limited and the sensitivity for interference in the limit region is quite high.